Heavy fuel oils can be considered as colloidal systems, in which asphaltenes with high C/H ratio are peptized as micelles in an oily phase. An important characteristic of colloidal systems, which distinguishes them from true solutions, is the presence of particles which are larger than molecules. The stability of a colloidal system typically depends on its ability to maintain the particles in solution and thus to prevent aggregation and precipitation. This stability in a fuel oil depends on the state of peptization, P, of the asphaltenes present, and the state of peptization depends in turn on both the peptizing (or solvent) power, P.sub.o, of the fuel oil medium and the peptizability (or solubility), P.sub.a, of the asphaltenes.
A publication, van Kerkvoort et al, Paper No. 229, IV Congres International du Chauffage Industriel, Paris, 1952, hereafter called the Reference, describes a method for evaluating the stability of fuel oils and the compatibility of fuel oil blends by mans of a Flocculation Ratio test. A procedure is given by the Reference for determining the minimum percentage aromatics that a test mixture of aromatic and paraffinic hydrocarbons should have which, when added to the fuel oil at a given dilution ratio, just fails to cause flocculation of the asphaltenes present in the fuel oil. This minimum percentage aromatics is called the Flocculation Ratio.
This Flocculation Ratio is determined in the Reference by making time consuming and laborious batch measurements involving a Spot Test, while this invention provides accurate method and apparatus for making the same measurements rapidly and continuously. The Flocculation Ratio (FR) is determined according to the Reference at different dilution ratios (DR) of the aromatic-non aromatic hydrocarbon mixture (e.g. toluene and n-heptane) and the fuel oil, after which a curve is obtained expressing the relation between the degree of dilution of the fuel oil and the minimum aromatic content (FR) that the aromatic-non aromatic hydro-carbon mixture should have in order to avoid asphaltene flocculation, that is, so that the asphaltenes are just peptized. The curve expressing this relation is preferably plotted as Flocculation Ratio versus the inverse of the dilution ratio, or FR versus 1/DR. It has been verified experimentally that such a plot is linear for a wide range of residual fuels and fuel oil blends. The linear plot versus 1/DR is preferred over the non-linear plot versus DR, since extrapolations can be made with better accuracy with the linear plot. An example of such a plot is shown in FIG. 1a, where DR is expressed as volume of diluent divided by mass of fuel. DR is sometimes expressed as volume of diluent divided by volume of fuel, but the conversion between these two forms of DR will present no problem.
An important and useful property of the FR versus 1/DR plot is that the intercept on the ordinate axis (FR.sub.max) and the intercept on the abscissa axis (DR.sub.min) provide the state of peptization, P, the peptizing power, P.sub.o, and the peptizability, P.sub.a, by the following formulas: EQU P.sub.o =FR.sub.max (DR.sub.min +1) EQU P.sub.a =1-FR.sub.max EQU P=P.sub.o /(1-P.sub.a)=DR.sub.min +1;
and furthermore, what is of particular practical significance when blending is involved, P.sub.o and P.sub.a are additive. Thus the stability/compatibility of a fuel oil blend can be calculated from the P.sub.o and P.sub.a values of the components used. For example, for a binary blend, the following equations are valid: EQU P.sub.o.sbsb.blend =V.sub.1 P.sub.o.sbsb.1 +V.sub.2 P.sub.o.sbsb.2 ( 1) EQU P.sub.a.sbsb.blend =(V.sub.1 M.sub.1 P.sub.a.sbsb.1 +V.sub.2 M.sub.2 P.sub.a.sbsb.2)/(V.sub.1 M.sub.1 +V.sub.2 M.sub.2) (2) EQU P.sub.blend =P.sub.o blend /(1-P.sub.a blend) (3)
where V is the volume fraction of each blending component and M its asphaltene content.
The physical significance of the quantities in the above equations is summarized:
FR: Flocculation Ratio: is the minimum aromatic content that an aromatic-non aromatic hydrocarbon mixture should have in order to dilute a fuel oil to DR volumes without causing flocculation of the asphaltenes.
DR: The number of volumes of dilution liquid per volume of fuel oil phase (asphaltene dispersion).
FR=f(DR): The curve represents the flocculation ratio (FR) as a function of the degree of dilution (with aromatic/non aromatic mixtures at different ratios); the curve gives the limiting conditions for a fuel oil at which the asphaltenes of an asphaltene dispersion are still peptized.
DR.sub.min : Is the maximum volume of non-aromatic hydrocarbon (FR=0) with which the fuel oil can be diluted without asphaltenes flocculation.
At infinite dilution of the asphaltene dispersion with an aromatic-non aromatic hydrocarbon mixture:
FR.sub.max : Is the aromatic content of the diluent liquid required to keep the asphaltenes peptized (at infinite dilution the peptizing power of the fuel oil medium is determined only by the diluent liquid).
1-FR.sub.max : Is the non-aromatic content at infinite dilution which can be tolerated without causing asphaltene flocculation.
P.sub.a : Is defined as the peptizability of the asphaltenes and is equal to 1-FR.sub.max. The better the peptizability of the asphaltenes, the higher 1-FR.sub.max will be.
P.sub.o : Is the peptizing power of the fuel oil medium and can be defined as the aromatic equivalent of this fuel oil expressed in volume percent of the aromatic component of an aromatic-non aromatic hydrocarbon mixture having the same peptizing power as the fuel oil.
P: Is the state of peptization of the asphaltenes in a fuel oil and is equal to P.sub.o /(1-P.sub.a), indicating that the state of peptization becomes better the higher the peptizing power of the fuel oil medium and the better the asphaltenes can be peptized. If P&gt;1 the fuel oil (blend) will remain free of dry sludge (stable fuel with asphaltenes peptized), otherwise (P&lt;1) the asphaltenes will flocculate (unstable fuel oil).